A sintered R-T-B-M magnet, including an R2T14B type compound as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances.
It has been known that if the rare-earth element R in the R2T14B phase of a sintered R-T-B-M magnet is replaced with a heavy rare-earth element RH (which may be Dy and/or Tb), the coercivity will increase. To achieve high coercivity even at a high temperature, a lot of such a heavy rare-earth element RH needs to be added.
However, if the light rare-earth element RL (which may be at least one of Nd and Pr) of the sintered R-T-B-M magnet is replaced with the heavy rare-earth element RH, the coercivity will certainly increase but the remanence will decrease instead. Furthermore, as the heavy rare-earth element RH is one of rare natural resources, its use is preferably cut down.
For these reasons, the coercivity of a sintered R-T-B-M magnet should be increased effectively with the addition of as small an amount of the heavy rare-earth element RH as possible and without decreasing the remanence.
Thus, a lot of people are now researching techniques for distributing, as effectively as possible, only a small amount of the heavy rare-earth element RH added to the structure of a sintered R-T-B-M magnet in order to increase the coercivity and minimize the decrease in remanence.
Patent Documents Nos. 1 and 2 teach distributing Dy in the vicinity of the grain boundary phase of a sintered magnet by making the sintered magnet from an alloy powder with a relatively high Dy concentration and an alloy powder with a relatively low Dy concentration. According to those patent documents, if Dy is distributed in the vicinity of the grain boundary phase of a sintered magnet, the magnetic properties will improve.
On the other hand, Patent Document No. 3 discloses a technique for diffusing a heavy rare-earth element RH (which is at least one element selected from the group consisting of Dy, Ho and Tb) inward through the surface of a sintered magnet body by heating the magnet body with the heavy rare-earth element RH supplied onto its surface.